FIGUEIREDO, IRINEU

"Magnetotelluric investigation in mountain chains of the Brazilian South-east: A crustal model proposal"

Process no. 238/92

Doctor - approved in 25/07/1997

Period - 1992-97

Dr. Sergio Luiz Fontes (adviser)

Dr. Antônio Lopez Padilha - INPE

Dr. Jandyr de Menezes Travassos – ON/CNPq

Dr. Luiz Fernando Santana Braga - PETROBRAS

Dr. Abel Gonzalez Carrasquilla -UENF

Abstract:

A magnetotelluric (MT) survey comprising 58 soundings along a 250 Km long profile was conducted crossing both the ‘Serra do Mar’ (RJ state) and ‘Serra da Mantiqueira’ (MG state) mountains chains in southeast Brazil. The frequency range covered was generally 0.003-300 Hz and the average spacing between soundings was about 4 Km. MT data show lateral variations from shallow depths that are limited by a good conductor at a 10 Km depth along the entire profile. The two-dimensional (2D) MT model is supported both by the surface geology (represented by the geologic units and associated faults) and by joint potential field 2D model ( gravimetric and magnetic) from available data for the region . The geo-electric structure seen in long period MT responses (>1s) are not captured by the 2D inversion due to both the smooth procedure (the rapid relaxation technique) employed in this study and possible 3D The depth to the top of the good conductor can be related both to a reported seismic transition velocity interface and to the 450 °C isotherm. These independent evidences suggest that a ruptile/ductile crustal transition may occur at the depth of about 10 Km. Na alternative explanation is related to the presence of meta-sediments at depths where the good conductor is found. However, no geological evidence for these meta-sediments are known to date in the studied area. Below the good conductor, it is possible to infer a Curie isotherm and the crust/mantle interface, based on magnetic and gravimetric data, respectively. Therefore the geophysical model proposed in this work covers the whole crust below the profile.